The natural gas found in coal is believed to have originated from the coal during its formation; and as such, coal is both the source and the reservoir rock. The natural gas in coal is typically composed of methane, more so than natural gases from other sources. Hence, this resource is commonly called coalbed methane.
Coal has the ability to hold large quantities of natural gas despite its low porosity. The reason for this large storage capacity is that the natural gas is stored as an adsorbed gas at near liquid density. This adsorption capacity is related to the fine pore structure of coal, where the majority of the porosity exists as micropores whose size is just slightly greater than molecular dimensions. These micropores result in a large internal surface area which can easily exceed 100 m.sup.2 /gm, and it is on this large surface area where the natural gas molecules are held by adsorption.
This fine pore structure is nearly absent in sandstone and carbonates. For example, a sandstone has an internal surface area closer to 1 m.sup.2 /gm. In these types of reservoirs, the natural gas is stored in less concentrated form as free gas. As a result, much greater porosities than those found in coal are required in sandstones or carbonates in order to store an equivalent amount of natural gas. For example, a 20 ft coal seam having a density of 1.5 gm/cc and a gas content of 500 SCF/ton contains over 13 BCF/section. A sandstone or carbonate of the same thickness would need a porosity of over 34% to have the same amount of gas-in-place at reservoir conditions of 1000 psia and 100.degree. F.
While gas is primarily stored in the micropores of the matrix, water is stored in the natural fractures of the coal--called cleats. It is through this cleat system that the microporous matrix is connected to a well drilled into the coal seam.
Usually, the coalbed methane production process begins by drilling at least one wellbore into the coal seam. At first a well typically produces water, contained in the cleat networks of the coal seam, and a small proportion of gas from the coal matrix. As the cleats are dewatered, the reservoir pressure near the wellbore is reduced. This lowering of reservoir pressure releases some gas from the surface of the coal. The gas migrates from the micropores of the coal matrix into the cleats. As water is produced from the coal, the water saturation in the cleats is reduced and the ability of the gas to flow in preference to water improves, i.e., the relative permeability to gas increases.
Most coal seams are also water aquifers. Consequently, an important consideration in a coalbed methane recovery project is the rate at which water migrates from the flanks of the coal seam into the coal cleats adjacent to the wellbore. In order to maintain or improve gas deliverability of a well, continuous production of fluids can be essential. If several wells in a field are shut-in for a considerable period of time, it is possible that water can invade the dewatered portions of the coal seam. Therefore, when the wells are put back on production, resumption of gas recovery at rates comparable to those achieved prior to shut-in may take considerable time and effort. The water influx to a coal well can have significantly reduced the gas relative permeability of coal during the shut-in period.
In commercial coalbed methane recovery projects, lack of demand for gas often forces operators to temporarily shut in some or all of the wells. Over time, the cleat networks in the coal adjacent the shut-in wells will be invaded with water originating from the flanks of the coal seam. As a result, the cleats in the coal adjacent to the wellbore have to be dewatered again before significant gas production resumes. Under some circumstances, it can take several months for the gas rates to return to the pre-shut-in production rates. Unfortunately, this lag period usually occurs when high gas rates are required to meet demand. If the demand for gas fluctuates routinely during the life of a coalbed methane recovery project, then shutting in wells during low demand and producing them during high demand can become a very inefficient method of operating a coalbed methane recovery project.
An alternative to shutting in the wells is to flare the excess gas. This has the desirable effect of keeping the cleat networks in the coal adjacent the well saturated with gas, but it has the undesirable effect of reducing total amount of natural gas available for sale, thereby wasting precious natural resources. There is a need for an alternative to shutting in wells during low demand for natural gas produced from coal seams without flaring the gas.
U.S. Pat. No. 4,544,037 to Terry discloses a method of initiating production of methane from wet coalbeds. The abstract states, "Rather than pumping water to lower the hydraulic head on the seam to permit desorption of methane within the coal, high pressure gas is injected into the seam to drive water away from the wellbore. Gas injection is terminated, and the well is open to flow". This patent does not disclose or suggest any method to handle fluctuations in gas demand in a coalbed methane project. Nor does it address means to minimize water influx during well shut-in.
In an article published in Ninth World Energy Conference Transaction, Vol. 2, 1975, pp. 103-118, the use of abandoned coal mines for gas storage is recommended. Although storing surplus gas in the void areas created in a coal seam after mining operations have been completed can be a feasible alternative to shutting in coalbed methane wells when gas demand is low, an abandoned coal mine may not be located close to a coalbed methane recovery project.
U.S. Pat. No. 4,623,283 to Chew discloses methods for preventing the introduction of water from a sandstone above the coal seam into a mine cavity from which combustion process gases are removed. All of the methods provide a barrier between the water sand and the mined coal cavity to prevent excessive water influx. The Chew patent does not disclose or suggest any techniques for inhibiting the migration of water within the coal seam itself during well shut-in.
There is a need for an efficient method of operating a coalbed methane recovery project when the demand for gas fluctuates during the life of the project without allowing the migration of water to invade the coal cleats adjacent to a wellbore. There is a need for an efficient method of producing gas from a coal seam at reduced rates during low demand without flaring the gas produced, and subsequently producing at high rates during high demand.